One thought, and this relates to FDA approval... once there is adult pfizer approval, a doc can do "off label" use of a drug. I.e. perhaps give an adult 30ug shot to a child. Will many pediatricians do that? I have no idea. Liability concerns, naturally. If you have 2 kids, maybe they could split a single 30ug dose (so 15 units, the 5-11 cohort study is 10ug) each. But my understanding is that could happen, completely legally, as soon as next week.
Now, as someone who may know someone who had a 6th grader who is the same size and development as many other kids in the same grade technically a few months older... I bet there are tens of thousands of folks who have already done it. Maybe one of those folks would come clean to their pediatrician and see if they would provide a reduced child size dosage for the second shot. That's all theoretical mind you. I have heard, sample size of 1, no ill effects were noted after 1 shot.
Additionally, with FDA approval off-label use would let doc give a J&J based person get a pfizer or moderna.